The invention relates to an arrangement in connection with a central lubrication system according to the preamble of claim 1, the arrangement comprising a lubricant vessel, a pump unit, a control unit, pipe systems, a pressure monitor unit, at least one feeder provided with at least one piston which moves due to the influence of the pressure of a lubricant present in the pipe system/object to be lubricated, and a movement monitor unit for each feeder in order to monitor the operation of the system, the lubricant being arranged to be pumped from the lubricant vessel along the pipe systems to the feeders and further to the objects to be lubricated, and a junction part located in the movement monitor unit outside a pressurized space.
Typically, conventional central lubrication systems known in the art include a lubricant vessel, a pump unit, an electric control unit, pipe systems, lubricant feeders and movement monitor units. The movement monitor units are conventionally used for monitoring movement of the piston of a feeder, and for controlling the operation of the system. In order to detect movement of the piston, the movement monitor unit comprises a switch. In the prior art, the switch is a magnetic switch to switch and release at a predetermined strength of a magnetic field, the field strengths of a switching point and a releasing point being different in magnitude, i.e. the switch involves hysteresis. Bodies of feeders are manufactured both from a magnetable (galvanized) and non-magnetable (acid-proof) material so that the body affects a change in the magnetic field caused by the piston, and thus the aforementioned operating points. In order to work well, such a solution would require a permanent magnet to be mounted to the piston so as to obtain a sufficient change in the magnetic field. However, the switch hysteresis restricts the use of a sensor in such a solution at a small dose, when the movement of the piston is small.
A problem with the above-described arrangement is that the piston moves in a space wherein large pressure variations (typically 0 to 250 bar) occur and, in addition, the diameter of the piston is small (typically 4 to 8 mm), so that it should be possible to detect even small movements of the piston. This conventional structure is not, however, capable of detecting movement of the piston with a sufficient accuracy at small movement lengths of the piston.